Optical measuring systems are known for determining the location and contour of complicated surfaces which have been machined, molded or otherwise fabricated. These devices typically use lasers and laser interferometers to determine the contour of the surface to be measured by taking a series of very accurate location measurements at a series of points on the surface in a controlled volume and converting the three-dimensional points into a representation of the contour being measured. Knowing three distances from a fixed reference location, the three-dimensional coordinates of the surface can be determined. Alternatively, determining one distance and two angles in the measuring system, the required three-dimensional coordinates can be obtained. In addition to measuring three-dimensional coordinates, these systems can also be used to measure merely two-dimensional coordinates such as the location of a target known to lie in a plane. Similarly, these systems can also be used to measure one-dimensional coordinates such as the distance or angular orientation of a target relative to a reference position.
In order to locate and trace the contour of the targeted surface to be measured, typically a retroreflector is utilized. This retroreflector is typically a corner cube or cat's eye retrosphere which is moved along the contour to be measured by hand or by robot machine and which constantly receives and reflects the laser beam from the optical measuring machine to provide a series of location readings for the contour of the surface being measured. Examples of these types of optical measuring systems are disclosed in the following U.S. patents, the disclosures of which are hereby incorporated herein by reference: U.S. Pat. No. 4,457,625 to Greenleaf et al; U.S. Pat. No. 4,621,926 to Merry et al; U.S. Pat. No. 4,714,339 to Lau et al; and U.S. Pat. No. 4,790,651 to Brown et al.
The target often used with a tracking laser interferometer is a spherically mounted retroreflector, which consists of a retroreflector whose center point is accurately placed inside a spherical ball. The ball is carried around the working volume and placed into contact with the surface to be measured as data therefrom is collected. The three-dimensional measurements made with such a tracker are actually made to the center point of the retroreflector, which will be offset by one ball radius from the surface the spherically mounted retroreflector is touching. The advantage of such a retroreflector is its ability to work with a variety of surface shapes at a constant offset.
However, there is a significant disadvantage to this type of retroreflector. The disadvantage is its size which is dictated by the size of the optical aperture needed which is from about 3/8" to 1" or larger. Thus, very small retroreflectors on the order of 1/32" or 1/8" cannot be used, and therefore, it is very difficult for such types of trackers to accurately measure small details in a surface such as grooves, holes, recesses, sharp inside radii, etc.
Examples of additional prior art devices relating to probes and measuring systems include the following U.S. patents: U.S. Pat. No. 2,589,618 to Learned; U.S. Pat. No. 4,926,559 to Knabel; U.S. Pat. No. 4,942,671 to Enderle et al; U.S. Pat. No. 4,972,597 to Kadosaki et al; and U.S. Pat No. 5,174,039 to Murai.
Thus, there is a continuing need to provide improved retroreflectors and retroprobes for use with dimensional measuring machines of versatility and high accuracy.